(1) Field of the Invention
The present invention relates to an image processing apparatus, an image forming apparatus, and an image processing method, and especially to an edge processing technique.
(2) Description of the Related Art
In practice employed in an image forming apparatus such as a digital color copier, an original image obtaining by reading a document is subjected to so-called edge enhancement processing in order to increase the density of edge areas (for example, boundaries between a character image and a background image) before such an image is formed on a recording sheet. Such edge enhancement processing is carried out by an image processing apparatus installed in an image forming apparatus in order to sharpen boundaries and thus to improve visibility of, for example, characters by widening the difference in density between edge areas and background.
A conventional image processing apparatus calculates the degree of variations in intensity (density) between a target pixel, i.e., a pixel currently subjected to processing, and pixels surrounding the target pixel. The target pixel is judged to be in an edge area when the calculation results in a value equal to, or greater than a predetermined threshold value. Each pixel judged to be in an edge area is processed so as to increase or decrease the density.
This processing sharpens a boundary between a character image and a background image, so that the character image becomes more legible.
Incidentally, recent development in reprography technology has resulted in higher-resolutions employed both at a reading end and output-end, which raises the following problem. When reading and outputting an image at high-resolution, an edge area of halftone image, such as a black outline against a gray background appears poorly in contrast. One specific example is an image of a black hair against the background of a dark-haired head highlighted under illumination.
When reading and outputting an image at relatively low-resolution, in most cases, the background image and the hair is properly distinguished and separated without loss of contrast. Now, as higher-resolutions become available, quality of the sort of images as described above has now started to be checked more severely.
The above-mentioned problem is ascribable to that a document image is reproduced too faithfully when processed at higher-resolutions. Owing to characteristics of human visual system, such type of images appear higher quality to human eyes, if edge areas (outlines) as above are enhanced (corrected) in some degree to be more prominent.
A conventional image processing apparatus, however, dose not determine pixels in outline areas of a halftone image as edge pixels because an intensity of such a pixel does not differ so greatly from the surrounding pixels. In addition, although a conventional image processing apparatus manages to judge such pixels as edge pixels, the problem remains unsolved. If edge pixels of a halftone image are subjected to similar enhancement processing the one for edge areas of a character image, the halftone edge areas are excessively enhanced, and thus the resulting image appears awkward, which only ends up in image quality reduction.